It is known that all metallic structures that come in contact with a medium having the properties of an electrolyte are susceptible to the phenomenon of corrosion. Such corrosion tends to destroy the metallic structure and, depending upon the particular corrosive conditions existing, destruction of the metallic structure may occur within a longer or shorter period of time. In many instances significant damage to the metallic structure may occur within a short period of time even though destruction of the metallic structure has not yet occurred.
There are a great many structures subject to corrosion damage, including bridges, pipes, storage tanks, reinforcing steel of concrete structures, structural steel and piles. In most cases the electrolytes for such structures comprise water with dissolved salts and moist soils.
Many techniques have been developed to minimize corrosion. Perhaps the most common method of minimizing corrosion of steel is painting. However, paint is not fully effective for underground and immersion conditions because of a gradual decrease in its resistance which may result from pin-holing and moisture permeation from the corrosive medium to the substrate metal. Corrosion protection in painted steel or steel containing structures is therefore often supplemented with another method commonly known as cathodic protection. Cathodic protection can also be used for unpainted surfaces.
As used herein, the term "cathodic protection" encompasses all manner of preventing or reducing corrosion of structures in electrolytes such as water, soil or chemical solutions using means which are at least partially electrical.
In general, cathodic protection systems operate by utilizing an electrical current to oppose a corrosion current between the structure being protected and an electrolyte. There are basically two known systems for generating opposing electrical currents, "sacrificial systems" and "impressed current systems." In sacrificial systems, the current is supplied by another metal which is galvanically more reactive than the metal of the structure. For example, metals such as aluminum, magnesium and zinc are galvanically more active than steel and are used as "sacrificial anodes" to protect steel structures. In impressed current systems, a consumable metal is used to drain DC current supplied from an external source into the electrolyte which will pass to the structure to be protected. The parts from which the current is drained are called "anodes" and the protected structure is called "cathode." In both sacrificial and impressed current systems of cathodic protection, a metallic path between the anode and the cathode is essential for flow of current to protect the structure.
The design of cathodic protection systems is influenced by numerous factors, including the type of metal to be protected, properties of the electrolyte (chemical, physical and electrical), temperatures, presence or absence of bacteria, shape of the structure, design life, constructability and maintainability. Cathodic protection has been achieved by application of various metallic and polymer webs, tapes, wires, ribbons and bars to a metallic structure being protected. U.S. Pat. No. 4,992,337 to Kaiser et al. (1991), for example, describes an improved arc spray process for applying metals or alloys comprising magnesium, zinc, lithium, and aluminum. The patent also references an article by H. D. Steffans entitled "Electrochemical Studies of Cathodic Protection Against Corrosion by Means of Sprayed Coatings" in Proceedings 7th International Metal Spraying Conference (1974) at p. 123, which describes the arc spray application and corrosion testing of zinc, aluminum, and zinc aluminum pseudo alloy coatings. Still further, U.S. Pat. No. 4,992,337 references an article by P. O. Gartland entitled "Cathodic Protection of Aluminum Coated Steel in Seawater" in Materials Performance June 1987 at p. 29, which reviews the arc spray coating of steel with aluminum 5 wt % magnesium, and summarizes the performance of the coating in seawater.
Cathodic protection using strips or bands of aluminum, zinc, magnesium or alloys thereof is described in U.S. Pat. No. 4,496,444 to Bagnulo (1985). Similarly, U.S. Pat. No. 5,411,646 to Gossett (1995) describes cathodic protection using a braided anode having a mixed metal oxide coating, and U.S. Pat. No. 5,547,560 to LeGuyader (1996) describes cathodic protection of steels and alloys in seawater using a saturated calomel electrode comprising an aluminum based gallium and/or cadmium alloy.
The use of foils in limited circumstances is also known in the field of cathodic protection., but none of the prior uses of foils is particularly viable. U.S. Pat. No. 5,167,352 to Robbins, for example, describes the construction of double wall tanks in which an outer wall envelope of aluminum foil is installed over a prefabricated tank. Robbins' use of aluminum foil is not self-supporting, and the physical strength for the foil is invariably augmented by a resinous coating applied after the installation is completed. The requirement of applying a coating after installation severely limits the applicability of the technology to relatively small tanks (less than 100 feet in diameter) because the tank must be fabricated and hydrotested before application of the aluminum foil envelope. The sequence of events required is: (1) filling the tank with water to check for leaks; (2) emptying the tank; (3) drying the tank interior to prevent inside corrosion; (4) wrapping the aluminum foil to the tank bottom to form an envelope; (5) providing temporary physical support to the foil during the formation of the envelope and lifting of the tank; (6) coating the aluminum foil, seal the overlaps, lift the tank, and position the tank on the foundation; and finally (7) removing the temporary physical support of the foil with sufficient care not to damage the foil and coating laminate.
Robbins' technology is of limited value for other reasons as well. Among other things, all overlaps of the aluminum foil must be completely sealed, as the aluminum foil is intended as a secondary containment. This greatly increases manufacturing difficulties. In addition, Robbins' technology cannot be utilized for existing above ground storage tanks which require replacement of corroded floors.
Other cathodic protection systems utilize wires and wire meshes in place of strips, bands and foils. U.S. Pat. No. 5,340,455 to Kroon et al. (1994), for example, a horizontally disposed cathodic protection anode is positioned between a membrane and the tank bottom, the anode being in the form of a matrix, maze or grid of electrically interconnected coated titanium wires or titanium clad copper wires, and such wires and titanium bars or ribbons. The wires are provided with a mixed metal oxide or noble metal coating. The bars or ribbons may also be coated. In lieu of the preferred titanium, other suitable metals may be used such as aluminum, tantalum, zirconium or niobium, and alloys thereof.
Still other systems alter the composition of the foundation. U.S. Pat. No. 5,174,871 to Russell (1992), for example, describes corrosion protection of underground structures using a high pH backfill including calcium silicate, calcium nitrate and a hydroxide such as calcium hydroxide or aluminum hydroxide.
In short, despite significant work invested over many years in the development of cathodic protection systems uniform protection is not generally possible with known systems. Among other things such systems continue to be problematic because:
1. The design calculations are performed with "assumed" resistivity of the electrolyte because the actual resistivity varies with time and pressure exerted by the tank bottom during operation which is not known during the design phase of the cathodic protection system:
2. The spacing of the anodes is influenced by the "assumed" resistivity; and
3. There is no proven design method to accurately predict the current distribution from the ribbon and wire systems on the tank plate. The accuracy of methods used to calculate the current distribution from the distributed anodes on the tank plate is questionable as well. If the "assumed" resistivity is not correct, adjustment of the system may not be possible.
Resistivities of the structure's foundation can range from 10,000 ohm-centimeters to 300,000 ohm-centimeters, and variations in resistivities are common from one location to another even within the same foundation. Galvanic anodes, when embedded in the foundation according to current technologies do not work satisfactorily because of high voltage drops between the anode and the structure. Impressed current anodes can be used in such high resistivity mediums, but they generate oxygen and chlorine gas during the chemical reactions, and these gases collect under the structure. Unless the oxygen and chlorine gases are completely purged with inert gas such as nitrogen, pitting corrosion occurs on the tank bottoms. Complete nitrogen purging and verification of its effectiveness is neither practical nor economical.
Still further, tests have shown that installation of impressed current cathodic protection designs should not be used in the annular space of double bottom storage tanks because oxygen that is generated by impressed current anode systems is retained within the closed systems and supports continued corrosion (Reference: Rials S. R. and Kiefer J. H., Conoco Inc, Evaluation Of Corrosion Prevention Methods For Above ground Storage Tank Bottoms. Materials Performance, National Association of Corrosion Engineers, Jan 1993).
Another problem with existing systems is potential damage to the anodes and anode connections after they are embedded in with the soil. To prevent settling of the structure, surrounding soil generally requires compacting, and compacting methods can potentially damage the anodes and anode connections. Compacting methods also affect the electrical resistivity which could be different from the electrical resistivity used in the design of the cathodic protection system.
These problems are particularly apparent when protecting structures such as petrochemical holding tanks because of the large surface area being protected, and difficulties associated with construction of the structures and foundations. Thus, there is still a need for improved cathodic protection systems.